As shown in FIG. 21, laser beam from a laser source 1 is converted into incident parallel beam by a collimator lens 2 and then enters a half mirror 3. The incident parallel light beam through the half mirror 3 is collected by an objective lens 4 into convergent light beam and irradiated on a recording or reflecting face 5.sub.1 of an optical disk 5 having preformed grooves provided therein. Light beam reflected on the recording face 5.sub.1 of the optical disk 5 is converted into reflected parallel beam by the objective lens 4 and then reflected by the half mirror 3 to reach respective detectors 6.sub.1 and 6.sub.2 of two-divided detector means 6.
A tracking control circuit 9 is adapted to output a tracking control signal so that the light quantities detected by the respective detectors 6.sub.1 and 6.sub.2 are equal to each other. Objective lens drive means 7 is adapted to drive the objective lens 4 in a radius direction of the optical disk (in a direction traversing the track of the optical disk) in accordance with the tracking control signal from the tracking control circuit 9.
As noted from FIG. 21, the incident parallel beam is a circular beam having a beam diameter of d.sub.1 while the reflected parallel beam is a circular beam having a beam diameter of d.sub.2. Accordingly, if the objective lens 4 is so set as to move in a direction of A or B within a range of the incident parallel beam, then the diameter d.sub.2 of the reflected parallel beam will be substantially equal to a diameter a of the objective lens 4. Also, the position of optical path of the reflected parallel beam will move in accordance with the movement of the objective lens 4.
The optical pick-up 8 includes the laser source 1, the collimator lens 2, the half mirror 3, the objective lens 4, the two-divided detector means 6 and the objective lens drive means 7.
The tracking control may be accomplished by selectively moving the whole optical pick-up 8 or only the objective lens 4 within the optical pick-up 8 in accordance with the distance of movement and/or the speed of movement thereof. In FIG. 21, only the objective lens 4 is shown to move.
FIGS. 7A and 7B illustrate distributions of phase and intensity of the incident parallel beam, respectively, while FIGS. 8A and 8B illustrate distributions of phase and intensity of incident parallel beam on the objective lens 4 when a center of the objective lens 4 is so positioned as to be coincident with a center of the incident parallel beam. As noted from FIG. 8B, the intensity distribution at that time is substantially of smooth gauss one having the maximum point provided at the center of beam and being zero at the position corresponding to the incident parallel beam interrupted by an objective lens holder 4.sub.1.
FIGS. 9A and 9B illustrate distributions of phase and intensity of the convergent beam into which the incident parallel beam is converted by the objective lens 4 when it irradiates the reflecting face 5.sub.1 of the optical disk 5 having no groove provided at a just-focus state thereof, respectively. As noted from FIG. 9B, the intensity increases only at the center of beam and declines near the center of beam although it decreases in a wave-like manner. Also, as noted from FIG. 9A, phase advancing portions (protrusion portions) and phase delaying portions (recess portions) are alternately provided in a concentric manner relative to the center of the beam. Furthermore, FIGS. 10A and 10B illustrate distributions of phase and intensity of the parallel light beam into which the reflected beam is converted by the objective lens 4. As noted from these figures, the distributions of phase and intensity of the reflected parallel beam are substantially identical to those of the incident parallel beam irradiating the objective lens 4.
FIGS. 11A and 11B illustrate distributions of phase and intensity of the convergent beam irradiating the reflecting face 5.sub.1 of the optical disk 5 having the grooves provided therein at just-focus and on-track states in the condition of the objective lens 4 being at the standard position where the center of the objective lens 4 is coincident with the center of the incident parallel beam. In FIGS. 11A and 11B, the various conditions are set as follows;
(1) wavelength of laser .lambda.=0.78 .mu.m PA0 (2) ratio of opening of objective lens NA=0.52 PA0 (3) ratio of land width relative to groove width=3:1 PA0 (4) depth of groove=.lambda./8 PA0 (5) .lambda./NA=1.5 .mu.m PA0 (6) track pitch p=1.4 .mu.m
As noted from FIG. 11B, the intensity distribution of the convergent beam irradiating the optical disk 5 is identical to that of the beam irradiating the reflecting face of the optical disk having no groove, but as noted from FIG. 11A, the phase distribution at its protrusion portions (the phase advancing portions) and at its recess portions (the phase delaying portions) is deformed by the effect of the grooves in the reflecting face thereof.
FIGS. 12A and 12B illustrate distributions of phase and intensity of the reflected parallel beam into which the reflected beam from the reflecting face of the optical disk 5 is converted by the objective lens 4, respectively. As noted from FIG. 12B, the intensity distribution of the reflected parallel beam has an arcuate recess portion g.sub.1 arcuately expanding in a rightwardly and leftwardly symmetrical manner relative to a center line l.sub.1 in comparison with the intensity distribution of FIG. 10B so that there are produced rightward and leftward protrusions r.sub.1 and r.sub.2. The arcuate recess portion g.sub.1 has the minimum width w.sub.1.
The center line l.sub.1 includes the center of the incident parallel beam and is drawn corresponding to a direction in which the grooves are formed. Thus, it will be noted that the center line l.sub.1 corresponds to the boundary line of the detected areas of the two-divided detector means 6. Therefore, as the detector 6.sub.1 detects the reflected light quantity q.sub.1 corresponding to the area D.sub.1 of the intensity distribution while the detector 6.sub.2 detects the reflected light quantity q.sub.2 corresponding to the area D.sub.2 thereof, the thus detected light quantities are equal to each other.
FIGS. 13A and 13B illustrate distributions of phase and intensity of light beam on the reflecting face 5.sub.1 of the optical disk 5 in case of the track pitch p being 1.6 .mu.m in the condition of the objective lens 4 being at the standard position while FIGS. 14A and 14B illustrate those of the reflected parallel beam at that case. As noted from FIG. 13A, the phase distribution at its protrusion portions (the phase advancing portions) and at its recess portions (the phase delaying portions) is deformed by the effect of the grooves in the reflecting face thereof. As noted from FIG. 14B, the intensity distribution of the reflected parallel beam has an arcuate recess portion g.sub.2 and an arcuate protrusion portion c.sub.1 formed in a rightwardly and leftwardly symmetrical manner relative to the center line l.sub.1.
As aforementioned, in case of the relation between the track pitch p and .lambda./NA being p&lt;.lambda./NA, the protrusions r.sub.1 and r.sub.2 are formed with the arcuate recess portions g.sub.1 disposed therebetween as shown in FIG. 12B, and in case of the relation between the track pitch p and .lambda./NA being p&gt;.lambda./NA, the arcuate protrusion portion c.sub.1 is formed as shown in FIG. 14B.
In the relation between the track pitch p and .lambda./NA being p&lt;.lambda./NA, the relation between the minimum width w.sub.1 and the track pitch p is expressed by the following expression wherein a is the diameter of the objective lens. EQU w.sub.1 /a=1-p/(.lambda./NA) (1)
FIGS. 15A and 15B illustrate distributions of phase and intensity of light beam on the reflecting face 5.sub.1 of the optical disk 5 in case of the track pitch p being 1.4 .mu.m in the condition of the objective lens 4 being at the standard position and in case of the irradiation point being offset by 0.1 .mu.m from the on-track position in one of the radius directions of the optical disk 5 while FIGS. 16A and 16B illustrate those of the reflected parallel beam at that case. As noted from FIG. 15A, the phase distribution at its protrusion portions (the phase advancing portions) and at its recess portions (the phase delaying portions) is deformed by the effect of the grooves in the reflecting face thereof. As noted from FIG. 16B, the intensity distribution of the reflected parallel beam has a protrusion r.sub.3 and a recess c.sub.3 formed between an arcuate recess portion g.sub.3. Therefore, the relation between the reflected light quantity q.sub.1 detected by the detector 6.sub.1 corresponding to the area D.sub.1 of the intensity distribution and the reflected light quantity q.sub.2 detected by the detector 6.sub.2 corresponding to the area D.sub.2 thereof is q.sub.2 &gt;q.sub.1. Thus, it will be noted that a difference between them indicates tracking error quantity.
FIGS. 17A and 17B illustrate distributions of phase and intensity of the reflected parallel beam in case of the track pitch p being 1.6 .mu.m in the condition of the objective lens 4 being at the standard position and also in case of the irradiation point being offset by 0.1 .mu.m from the on-track position in one of radius directions of the optical disk 5. As noted from FIG. 17B, the intensity distribution of the reflected parallel beam has an arcuate recess portion g.sub.2 and an arcuate protrusion portion c.sub.1 formed in a rightwardly and leftwardly symmetrical manner relative to the center line l.sub.1, respectively and a protrusion r.sub.4 and a recess c.sub.2 formed with the portions g.sub.2 and c.sub.1 disposed thererbetween.
FIGS. 18A and 18B illustrate distributions of phase and intensity of the incident parallel beam on the objective lens 4 when the center of the objective lens 4 is offset by 200 .mu.m in one of the radius directions of the optical disk 5 from the center of the incident parallel beam. This condition corresponds to the condition in which the center of the objective lens 4 is so controlled as to move by 200 .mu.m in one of the radius directions of the optical disk 5 from the standard position coinciding with the center of the incident parallel beam. A center line l.sub.2 of FIG. 18B includes the center of the objective lens 4 and is drawn in a direction in which the grooves are formed.
As noted from FIG. 18B, the intensity distribution at that time is substantially of smooth gauss one having the maximum point offset by 200 .mu.m from the center of the objective lens 4.
FIGS. 19A and 19B illustrate distributions of phase and intensity of the convergent beam on the reflecting face 5.sub.1 of the optical disk 5 having the grooves formed therein in case of the center of the objective lens 4 being offset by 200 .mu.m in one of radius directions of the optical disk 5 from the center of the incident parallel beam and also in case of the convergent beam irradiating the reflecting face 5.sub.1 of the optical disk 5 at the state of just-focus and on-track under the aforementioned setting condition. FIGS. 20A and 20B illustrate distributions of phase and intensity of the reflected parallel beam into which the beam reflected on the reflecting face 5.sub.1 of the optical disk 5 is converted by the objective lens 4, respectively. A center line l.sub.1 of FIGS. 20A and 20B corresponds to the boundary line of the detecting areas D.sub.1 and D.sub.2 for the detectors 6.sub.1 and 6.sub.2 of the two-divided detector means 6.
FIG. 6A shows a cross-sectional face of the intensity distribution of FIG. 20B in a solid line. As noted from this figure, the arcuate groove portion g.sub.3 moves in a direction of the area D.sub.2 and the intensity corresponding to a protrusion r.sub.6 increases relative to that corresponding to a protrusion r.sub.5. This is caused by the position of the maximum intensity distribution of the incident parallel beam incident on the objective lens 4 shown in FIG. 18B being offset from the center of the objective lens 4. Estimating a ratio of the reflected light quantities q.sub.1 and q.sub.2 detected by the detectors 6.sub.1 and 6.sub.2, q.sub.1 /q.sub.2 =1/1.23 is obtained and there occurs a difference between the reflected light quantities detected by the detectors 6.sub.1 and 6.sub.2 in spite of the on-track condition.
FIG. 6A shows a cross-sectional face of the intensity distribution of light beam in case of the center of the objective lens 4 being offset by 200 .mu.m in the other radius direction of the optical disk 5 from the center of the incident parallel beam in a dotted line. A ratio of the reflected light quantities q.sub.1 and q.sub.2 detected by the detectors 6.sub.1 and 6.sub.2 is q.sub.1 /q.sub.2 =1.23/1 and there occurs a difference between the reflected light quantities detected by the detectors 6.sub.1 and 6.sub.2 in the same manner.
It will be noted that the prior tracking control tends to produce the difference between the reflected light quantities detected by the respective detectors of the two-divided detector means in spite of the irradation point being in the condition of on-track when the objective lens within the optical pick-up moves in the radius direction of the optical disk. Thus, an accurate tracking error information cannot be advantageously obtained.